Course:CONS200/2021/Rodent invasion in New Zealand: The lasting impacts on ground birds

From UBC Wiki

Background on the species invasion in New Zealand

A rat attacking a ground bird

The ground bird is also widely known as flightless birds that have lost the ability to fly through evolution. [1] The two main factors that distinguish between flying birds and flightless birds are that flightless birds [1]have relatively small wing bones and have no keel on their breast bones or have degenerated. [2](The keel anchors the muscles required for the movement of the wing.) Ground birds have always played the role of ecosystem engineers in this area of ​​New Zealand due to the physical interference of their frequent nesting and burrowing.[3]However, the characteristic of flightlessness makes this flightless group more vulnerable to persecution by invasive species.The population of New Zealand ground birds is declining worldwide, and their impact on the community and ecosystem level is mainly caused by invasive predatory rodent.[4] The direct and indirect effects of the invasion of foreign rats have produced many different results on the lives of different birds and ecosystems on the islands.[3]In essence, the direct and indirect impacts of the reduction in the number of ground birds on the ecosystem are complex.[3]

Initially, there were no native rats in New Zealand before, but then three highly invasive rodents broke into the island: Rattus Eulans, Rattus rattus, and Rattus norvegicus, which have long-term impacts on New Zealand's native ground birds.[5]Since these rodents are omnivore mammals, they can eat almost everything, especially ground birds and eggs.[5]

Studies have shown that the invasion of foreign rats reduces the fertility of forest soil, leaves and If garbage and reduces the decomposability of waste by destroying nutrient subsidies derived from ground birds. [3] The Invasive rodents have had a severe impact on the structure of the invertebrate food web of ground birds on New Zealand Island. These rats will affect the number of bird populations by eating eggs, chicks and adults.

According to the research, Kākāpō, Kiwi bird, and Kākā are the three endangered bird species which are severely affected by the invasive rodent in New Zealand.[6]Fortunately, New Zealand implemented timely interventions for invasive alien species, and it is now widely regarded as the world leader in eradicating rodents on the island.[7]

Threats to the native bird

Rats have an impact on vegetation by extirpating burrowing seabirds by eating their eggs, chicks, and adults.[8] Between rat-invaded and uninvaded (seabird-dominant) islands, ten network topological metrics (including entropy, generality, and vulnerability) were examined.[9] We discovered that invertebrate food webs on rat-infested islands were smaller and less complicated than those on seabird-dominated islands, which could be owing to the suppression of seabird-derived nutrients and the resulting impacts on trophic cascades.[9]

Invasive rodent in New Zealand

Polynesian rat
Rattus Eulans

Kiore mice are also known as Polynesian rats, first introduced to New Zealand by Polynesians around 1250-1300 AD.[10]Polynesian rats resemble other rats in appearance, such as black and brown rats. It has large round ears, a pointed nose, black or brown hair, a lighter abdomen, and relatively small feet. Its body is thin and long, up to 6 inches (15 centimeters) from nose to bottom of tail, making it slightly smaller than other rats related to humans. It is usually characterized by a black upper margin of the hind foot near the ankle. Kiore's invasion directly caused the elimination of snipes, owls, nightingales, petrels, and giant lizards on the New Zealand mainland.[11]The species decline is that in New Zealand and its offshore islands, many birds live without predators of terrestrial mammals, so they have not formed a behavioral defense against mice in the process of evolution. [11]However, due to the invasion of more aggressive European rodents; and the New Zealand Department of Conservation has implemented plans to eliminate Polynesian rats on most offshore islands within its jurisdiction, and other conservation organizations have adopted similar procedures in other protected areas, it replaced Kiore's dominance and replaced them.  Therefore, few Kiore mice survive on the New Zealand mainland now. [11]

Ship rat
Rattus rattus

Europeans introduced ship rats (Rattus rattus), but they were not established until the 1860s.[12] The black rat is a larger murine animal, with a head length of 14-23 cm, a tail of 17-28 cm, and a weight of 75-230 grams. It has a smaller and slimmer body shape and a sharper beak than a brown mouse. Their ears are relatively long, and another feature is that their head and body are usually longer than their naked tail. [13]Nowadays, ship rats may be found throughout the nation. These rats vary from other species in that they prefer to live and breed on trees. Because the invasion rats devour native species and their eggs significantly influence New Zealand's wildlife.[12]When they were mistakenly introduced to Big South Cape Island (off Stewart Island) in 1964, they soon wiped off five native bird species, one bat species, and a big flightless weevil.[12]In a study at Kwai Bus, ship rats damaged and stole nestlings from 16 percent of tiny bird nests. Norway rats like to live on the ground, mainly near wetlands or in moist lowland vegetation.[12] They endanger the lives of birds by breeding close to the ground and stealing their eggs.

Brown rat
Rattus norvegicus

The brown rat (Rattus norvegicus) first arrived before 1800, and the brown rat posed a severe threat to much native wildlife in New Zealand.[14]The fur of the brown rat varies from taupe to black, with a pale underside and a long, sparsely hairy tail. The body length is generally 30-50 cm, and the tail length is less than the body length, averaging 15-25 cm. The average weight is between 200 and 500 grams.[15][16] In the North and South islands harbours, house mice (Mus musculus) reside in every piece of the shrub. House mice deprive many natural ground-feeding food species by consuming insects, fallen seeds, and berries.[14]Every two or three years ('masting' years), beech trees produce large seed yields, offering a feast for mice, whose populations often skyrocket in response.[14] Mice, in turn, provide food for stoats, whose numbers are expected to rise as well. After the seeds have germinated, withered, or been eaten, and many of the mice have been devoured, a predatory stoat population switches its predatory focus to local birds. [14]

Invaded ground bird in New Zealand

Today only sixteen flightless species remain: one parrot, two rails, five ratites (all kiwi), two teals and six penguins. A further fifteen flightless birds are known to be extinct: eleven ratites (all moa), three rails and a wren.[6]


The kākāpō is a large, round parrot; it is 59-64 cm long and weighs 2-4 kg. It has a pair of relatively short wings and lacks the keel bones that control the flight muscles of birds. [17]Its wings are not enough to make it fly, but only as a general balance or provide a certain degree of support when jumping from a tree.[17]Unlike other birds, the kākāpō has to maintain a light body.They tend to store a lot of fat in their body, making it the most weighty of their kind.[17]

kākāpō are prey for rats and stoats, while their eggs and chicks are vulnerable to rodents. Females incubate eggs and rear babies on their own.[18] When the nest is unattended, eggs and chicks are more susceptible to predation because they must spend extended periods away from the nest foraging.[9]Chick raising takes a long time, and nests grow stinky and are easy to discover for predators. When they're not in their nests, kākāpō usually freeze and rely on their cryptic coloration to keep predators at bay. While this method worked well when the significant predators were birds hunted by sight, it is entirely worthless when the main predators are mammalian predators that hunt by scent.[19]

Many Mustelidae animals (such as ferrets and different species of the Mustelidae) have been introduced to this land of New Zealand to reduce the number of invasive species of hares. Still, they have also disturbed the original ecosystem, and most native species such as kakapo have become prey targets.[20]Some herbivorous species, such as deer, compete with kakapo for food and have even wiped out some of the kakapo's favourite plants.[20] The kakapo population has been steadily declining since Polynesian.

Little-spotted kiwi

Kiwi birds are flightless birds endemic to New Zealand.[21]The kiwi bird lives up to 50 years, and 30 years in the zoos. The largest size is brown kiwi, the height is around 20 to 25 inches. The smallest size is the little spotted kiwi, which has a height between 14 to 18 inches.[22]Kiwi birds have hairlike feathers, short and stout legs, and kiwi birds have nostrils at the end of their long beak to detect prey. The egg of kiwi birds is one of the largest in comparison to body size, it reaches up to 20 percent of the mother weight. [22]

One of the reasons for decreasing kiwi species is habitat loss through intensive forest clearance to establish agricultural land.[23]The main reason is the introduction of mammalian predators. The agents of decline are stoat (Mustela erminea), ferret (Mustela furo), weasel (Mustela nivalis vulgar), feral cat (Felis catus), dog (Canis familiaris), pig (Sus scrofa) and possum (Trichosurus vulpecula). [24][23][25][26][27][28]As the most important part, rats are one of the most dangerous kiwi birds.


Kākā is classed as “endangered” parrots found in large forests in New Zealand.[29]It is a close relative of kākāpō and they are both from the family of “Strigopidae”. But it is easy to distinguish kākā from the smaller body size.[7]

The kākā populations have been declining due to the competition and predation from invasive rodents.[30]Possums share similar food with kākā and remove high energy food resources which kākā need for maintaining life. The predation on kākā by rats is another significant reason for the declining parrot.[31]Since kākā is likely to build the nest in the deep hole, it leaves no room to escape from predators. The nest also has a solid and distinct smell, which makes kākā vulnerable to the rats.[31]

Conservation Activities

Relationship between ground birds and rats

In the past ten years, significant progress has been made in multi-species eradication, including rodents, community engagement, rodent eradication, transfer of technology to continental ecological reserves, and development of new rodent management tools.[8] Today, bird conservation is a strong driver for prioritizing rodent eradication, but robust quantitative estimates of impacts are needed to ensure that management actions are cost-effective.[4]

Invasive rats have a significant impact on island habitats through direct and indirect effects toward plants. Eating eggs, chicks and adults is a way that rats affect vegetation by extirpating burrowing seabirds.  The native ground bird and rats are like the protector and invader in the system. Birds act as ecosystem engineers, influencing plant communities by burying and trampling seeds and seedlings, also by altering microclimates. Rats act as the invader which directly affect plant communities by ingesting seeds and seedlings, which was protected by the seabirds.[8]

In areas with high density of seabird burrows, seedling density was low, especially for the smallest seedlings. Microclimate change had the greatest effect on seedling species richness and diversity, but had little effect on seedling density. Island deratization or low rodent population had the lowest diversity and richness of seedlings (and adults), but the highest seedling density. The seedling communities on these islands are dominated by Pseudopanax Lessonii and Coprosma macrocarpa. This suggests that lasting effects of rats may prevent the island from returning to its pre-invasion state.[8]

Eradication of rodent

Invasive predators have been exterminated from 10% of New Zealand's offshore islands, and the country now has a plan to eradicate them from the entire country. [19] examine New Zealand's present understanding of invasive predator ecology and control strategies, biological research, technological advancements, social capability, and enabling necessary policies.[19] We go over the economic costs and benefits before proposing a 50-year plan for a predator-free New Zealand, ecologically feasible, socially desired, and commercially viable.[17]Invasive predator elimination from the two largest offshore islands, mammal-free mainland peninsulas, giant eco sanctuaries, and hundreds of local projects that will combine eradication and control concepts on landscape scales are part of the proposal.[19]

Using brodifacoum to eradicate rats from Kapiti Island

On Kapiti Island (1965 hectares) and its tiny outlying islands, an eradication operation against two species of rats (Rattus norvegicus and Rattus exulans) was carried out in 1996.[32]Non-toxic bait trials were conducted to identify the hazards to non-target species, and research was undertaken to acquire baseline data for monitoring the reaction of flora, invertebrates, reptiles, and birds to rat removal. In September and October, Talon 7-20 bait (containing 0.002 percent brodifacoum) was dispersed by helicopter and handed over Kapiti Island, with bait stations on the outer islands.[32]The effects on birds and reef fish were studied. Although the poisoning operation resulted in the deaths of non-target birds, post-eradication monitoring revealed that the toxin had a minor negative impact on breeding and that the recruitment of new members would quickly make up most losses into the breeding population.[32]


The number of New Zealand ground birds is declining globally. Undoubtedly, their impact on communities and ecosystems has become unignorable. The invasion of New Zealand had a considerable impact on ground birds. With the invasion of Rattus Eulans, Rattus rattus and Rattus norvegicus, these three rodents have directly caused a severe long-term effect on the life of New Zealand’s native ground birds.[5] As rats significantly infect the species of ground birds, there are only 16 species left. Kakapo, kiwi and Kaka are the three main species most infected.[6]The protection plan for endangered bird species has also become the top priority of the protection department. Targeted methods include examining New Zealand’s current understanding of invasive predator ecology and control strategies, biological research, technological advancement, social capabilities, and enabling necessary policies.[19]Moreover, according to research investigations, significant progress has been made in the extinction of multiple species in the past ten years, including rodents, community participation, rodent control, and the development of new rodent management tools.[8][5]



  1. 1.0 1.1 1.2 Powlesland, R (2006). "A parrot apart: the natural history of the kakapo (Strigops habroptilus), and the context of its conservation management/by Ralph G Powlesland, Don V Merton and John F Cockrem". Notornis. 53: 3–26.
  2. 2.0 2.1 McClelland, P. J (2002). ". Eradication of Pacific rats (Rattus exulans) from Whenua Hou Nature Reserve (Codfish Island), Putauhinu and Rarotoka Islands, New Zealand". Turning the tide: the eradication of invasive species.: 173–181 – via IUCN SSC Invasive Species Specialist Group, IUCN, Gland, Switzerland and Cambridge,.CS1 maint: extra punctuation (link)
  3. 3.0 3.1 3.2 3.3 3.4 Walker, Susan (2019). "Spatial patterns and drivers of invasive rodent dynamics in New Zealand forests". Biol Invasions 21: 1627–1642.
  4. 4.0 4.1 4.2 Moors, P J (May,2010). "Reducing the rat threat to island birds". Bird Conservation International. 2(2): 91–114. Check date values in: |date= (help)
  5. 5.0 5.1 5.2 5.3 5.4 Roy, Eleanor Ainge (2016). "No more rats: New Zealand to exterminate all introduced predators". The Guardian: 0261–3077.
  6. 6.0 6.1 6.2 6.3 "Flightless birds of new zealand". Nature (London). 164(4182): 1077–1078. 1949.
  7. 7.0 7.1 7.2 Rauzon, MJ (2007). "Island restoration: exploring the past, anticipating the future". Marine Ornithology. 35: 97–107.
  8. 8.0 8.1 8.2 8.3 8.4 8.5 Grant-hoffman, M (2010). "Invasive rats alter woody seedling composition on seabird-dominated islands in new zealand". Oecologia. 163(2): 449–60.
  9. 9.0 9.1 9.2 Brockie, B (2007). "Introduced animal pests - Rats and mice". Te Ara - the Encyclopedia of New Zealand. Archived from the original on |archive-url= requires |archive-date= (help). Retrieved 22 October 2021.
  10. 10.0 10.1 Lentini, P.E (2018). "Using fossil records to inform reintroduction of the kakapo as a refugee species". Biological Conservation. 217: 157–165.
  11. 11.0 11.1 11.2 11.3 "Polynesian rat". Wikipedia.
  12. 12.0 12.1 12.2 12.3 12.4 "Course:CONS200/2021/Impacts of Climate Change on Pacific Salmon in the Fraser River".
  13. 13.0 13.1 [ "Black rat, House Rat, Roof Rat, Ship Rat (Rattus rattus)"] Check |url= value (help). 19 October 2016.
  14. 14.0 14.1 14.2 14.3 14.4 Atkinson, I (1973). "Spread of the Ship Rat (Rattus r. rattus L.) in New Zealand". Journal of the Royal Society of New Zealand. 3: 457–472.
  15. 15.0 15.1 Kurta, A (1995). "Mammals of the Great Lakes Region". University of Michigan Press. Michigan, USA.: 186–188.
  16. Twigg, G (1975). The Brown Rat. David & Charles Inc. New Pomfret, Vermont, USA.
  17. 17.0 17.1 17.2 17.3 Ruffino, Lise (summer 2014). "Assessment of invasive rodent impacts on island avifauna: methods, limitations and the way forward". Wildlife Research: 185–195. Check date values in: |date= (help)
  18. 18.0 18.1 Lentini, P. E (2018). "Using fossil records to inform reintroduction of the kakapo as a refugee species".
  19. 19.0 19.1 19.2 19.3 19.4 Gibbs, George (2008 12/17). "The end of an 80-million year experiment: a review of evidence describing the impact of introduced rodents on New Zealand's 'mammal-free' invertebrate fauna". Springer Link. Archived from the original on 6,5,2018. Check date values in: |date=, |archive-date= (help)
  20. 20.0 20.1 20.2 Clout, M (1998). "Saving the Kakapo: the conservation of the world's most peculiar parrot". Bird Conservation International. 8(3): 281–296.
  21. 21.0 21.1 Powlesland, R.G (1992). "Breeding biology of the kakapo Strigops habroptilus on Stewart Island". New Zealand. Ibis. 134(4): 361–373.
  22. 22.0 22.1 San Diego Zoo (October 6 2008). "Birds: Kiwi". Check date values in: |date= (help)
  23. 23.0 23.1 23.2 HOLZAPFEL, S. "2008) Kiwi (Apteryx spp.) recovery plan". 2008-2018. Kiwi (Apteryx spp.) recovery plan, 2008-2018: 71.
  24. 24.0 24.1 BASSE, B (1999). "Analysis of the impact of stoats (Mustela erminea) on northern brown kiwi, (Apteryx mantelli)". in New Zealand. Wildlife Research. 26: 227–237.
  25. 25.0 25.1 MCLENNAN, J (1996). "Role of predation in the decline of kiwi (Apteryx spp.) in New Zealand". New Zealand Journal of Ecology. 20: 27–35.
  26. 26.0 26.1 PIERCE, R (2003). "Call count responses of North Island brown kiwi to different levels of predator control in Northland, New Zealand". Biological Conservation. 109: 175–180.
  27. 27.0 27.1 MURPHY, E (2008). "Diet of stoats at Okarito Kiwi Sanctuary, South Westland, New Zealand". New Zealand Journal of Ecology. 32: 41–45.
  28. 28.0 28.1 Metzler, C (2011). "Home ranges and dispersal patterns of Great spotted Kiwi (Apteryx haastii) subadults" (PDF). Home ranges and dispersal patterns of Great spotted Kiwi (Apteryx haastii) subadults.
  29. 29.0 29.1 Beggs, J (1987). "Energetics of South Island kaka (Nestor meridionalis rneridionalis) feeding on the larvae of kanuka longhorn beetles (Ochrocydus huttoni)". New Zealand Journal of Ecology. 10: 143–147.
  30. 30.0 30.1 Beggs, J. "Energetics of kaka in a South Island beech forest". MSc thesis, University of Auckland.
  32. 32.0 32.1 32.2 32.3 EMPSON, RAEWYN (1999). "The risks, costs and benefits of using brodifacoum to eradicate rats from Kapiti Island, New Zealand" (PDF). New Zealand journal of ecology: 241–254.
  33. Thoresen, J (2017). "Invasive rodents have multiple indirect effects on seabird island invertebrate food web structure". Ecological Applications. 27(4): 1190–1198.
  34. Calder, W. A (1978). The kiwi. Scientific American. pp. 239(1).
  35. HOLZAPFEL, S (2008). "(2008) Kiwi (Apteryx spp.) recovery plan". 2008-2018. Kiwi (Apteryx spp.) recovery plan: 71.
  36. Beggs, J (1991). "The kaka (Nestor meridionalis), a New Zealand parrot, threatened by introduced wasps and mammals". Biological Conservation. 56: 23–38.
  37. Animals Network Editors (4 Nov. 2017). "Antelope". Animals.Net. Check date values in: |date= (help)